The introduction of olefin polymers as insulation about wires and cables was an important development in the wire and cable industry. Olefin polymers were found to be corrosion resistant, abrasion resistant and to afford protection to the wires and cables about which they were extruded. Although olefin polymers have properties which make them desirable for use as insulation material, they also have other properties which presented problems. For example, olefin polymers were found to degrade upon aging. Consequently, antioxidants were added to olefin polymers in order to extend their working lives.
A problem which is still of concern to the wire and cable industry, however, is the susceptibility of olefin polymers to water treeing and electrical treeing. Water trees occur in a polymer when the polymer is subjected to an electrical field over extended periods of time while in a water environment. Water trees are named as such because of the resemblance to branched trees. In actuality, however, water trees are fine passages or voids in the polymers which are visibly white when filled with water, but are invisible when dry. For purposes of observation, polymer samples having water trees must be boiled in water or dyed.
It is known that water trees grow in polymers from points in which there are voids or extraneous matter such as metal. This is particularly true in the case of an insulated electrical cable which is subjected to high voltages while in a water environment. Nevertheless, it is difficult to keep extraneous matter out of the polymeric insulation in order to prevent voids from occurring, in spite of continued efforts to improve production processes.
In contrast, electrical trees, which are simply called trees, are visibly black because they grow as a result of electrical discharges that carbonize the polymeric material, leaving carbon therein.
The problems of water trees and electrical trees have been generally approached from the standpoint of adding various additives to olefin polymers. This expedient, in many cases, has given rise to other problems due to the nature of the additives used.